The present invention relates to a vacuum valve of the type having a valve stem and a magnetic drive.
Electromagnetically operating vacuum valves are known in which the electromagnetic drive acts only in the direction of opening, so that, in the event of a power failure, the valve is brought into the secure closed position. In the case of such electromagnetically driven vacuum valves it is further known to switch, for the purpose of holding open the valve in the stationary position, by switching the coils over to a holding-open force which is significantly lower with respect to the opening force. This prevents the generation of excessive temperatures during the stationary opening operation which, as a rule, in single-coil implementations is realized by means of a protective mechanism against excessive temperatures.
Magnetic drives for vacuum valves acting in the opening and closing direction are the exception rather than the rule, since valves operated magnetically only in the opening direction and returned into the closed position mechanically by means of springs entail the advantage that in the event of power failure the valve is brought into the secure closed position.
The service life of such vacuum valves is limited to approximately 200,000 actuation cycles. This short service life is due, in particular, to chatter phenomena during the electromagnetic movement into the closed position, as well as through the spring-actuated movement into the closed position. It must be taken into consideration that between the actuated valve stem/valve disc and housing, a metal bellows is conventionally disposed. This bellows encapsulates the closure spring elements and frictional seals on the valve stem, against the vacuum atmosphere. Reverberations of the bellows are a problem.
Prevention of this problem is the goal of the technique disclosed in EP-A 0 264 466. Reverberations of the bellows especially limits the service life of the valve.
Regarding the prior art with respect to magnetically actuated vacuum valves, reference is furthermore made to CH-A 653 103 which describes a pneumatic regulation device for a vacuum valve, EP-A 0 219 572 which describes a combination of electric lifting solenoid and permanent magnets, DE-A 3 704 504 which describes an electromagnetically actuated pipe joint valve, and EP-A 0 173 115 which also discloses a solenoid valve for vacuum applications.